1. Field of the Invention
The present invention relates to a changeover circuit in a tape recorder.
2. Description of the Related Art
In many cases, a home-use tape recorder has a single head used as a recording head and a reproducing head. A circuit arrangement for such a common use is shown in FIG. 1, for example.
More specifically, reference numeral 10 denotes a recording/reproducing head for a tape recorder, and reference numeral 20 denotes a recording/reproducing integrated circuit (IC). In this case, the IC 20 has a recording circuit and a reproducing circuit which are integrated in one chip IC as a whole. Reference numeral 21 denotes an output amplifier at the final stage of the recording circuit; a reproducing equalizer amplifier 22 at the first stage of the reproducing circuit; and, a changeover switch 23 for switching recording/reproducing.
The IC 20 has external connection terminals (pins) T21 to T24. A power supply voltage VCC is applied to the terminal T21, and a capacitor C51 for high-frequency peaking in a reproducing state is connected to the terminal T22. A bypass capacitor C52 is connected to the terminal T23, and the terminal T24 is grounded.
A voltage forming circuit 24 is arranged in the IC 20. In the forming circuit 24, a DC voltage VBIAS (=VCC/2) which is 1/2 the power supply voltage VCC of the terminal T21 is generated. The voltage VBIAS is applied to the amplifiers 21, 22 and so on as a reference or bias voltage thereof, and is output to the terminal T23.
Reference numeral 30 denotes an oscillation circuit for an AC bias in a recording state, and reference numeral 40 denotes a changeover circuit for switching a signal line between a recording state and a reproducing state. In this case, the oscillation circuit 30 has an oscillation transformer M31, and an oscillation capacitor C31 is connected to the secondary coil L31 in parallel. The changeover circuit 40 has switching transistors Q41 and Q42 and transistors Q43 and Q44 for driving the changeover circuit 40. A voltage V41 higher than the voltage VBIAS is applied to a terminal T41, and a switching signal S42 which is set to "L" in a recording state and to "H" in a reproducing state is supplied to a terminal T42.
Since S42="L" is set in a recording state, the transistor Q44 is turned off to turn off the transistor Q43, thereby turning off the transistors Q41 and Q42. That is, the transistors Q41 and Q42 are equivalently disconnected.
In addition, in the recording state, the changeover switch 23 is connected in the state shown in FIG. 1, and a recording audio signal is extracted from the output amplifier 21. The audio signal is supplied to the head 10 through a signal line formed of the amplifier 21, the changeover switch 23, the terminal T22, the head 10, the parallel circuit formed of the recording equalizer capacitor C53 and a resistor R51, a parallel circuit formed of coil L31 and the capacitor C31, the capacitor C52, the ground, and the terminal T24.
At this time, an AC bias signal is extracted from the coil L31 of the transformer M31 of the oscillation circuit 30, and the AC bias signal is supplied to the head 10 through a signal line formed of the coil L31, the elements C53 and R51, the head 10, the capacitor C51, the ground, the capacitor C52, and the coil L31.
Therefore, the audio signal from the amplifier 21 is AC-bias-recorded on a tape (not shown).
Since S42="H" is set in the reproducing state, the transistor Q44 is turned on to turn on the transistor Q43, thereby turning on the transistors Q41 and Q42. In the reproducing state, the changeover switch 23 is connected in a reverse state of the state shown in FIG. 1.
Therefore, the lower end portion of the recording/reproducing head 10 in FIG. 1 is connected to the IC 20 through a signal line formed of the head 10, the transistor Q41, Q42, the capacitor C52, the ground, and the terminal T24 in this order. For this reason, a reproducing signal from the head 10 is supplied to the amplifier 22 through a signal line constituted by the head 10, the terminal T22, the changeover switch 23, and the amplifier 22.
Therefore, the reproducing signal from the head 10 is subjected to a reproducing equalization process or the like to be supplied to the circuit at the following stage.
In this manner, in the tape recorder shown FIG. 1, the head 10 is used as recording and reproducing heads by the changeover switch 40.
Since a DC potential at the output terminal of the amplifier 21 and a DC potential at the input terminal of the amplifier 22 are generally set to VCC/2, a DC potential at the terminal T22 is also set to VCC/2. However, the voltage VBIAS is output to the terminal T23, and VBIAS=VCC/2 is established. Though, even if the head 10 is directly connected to the terminal T22 (even if a capacitor for cutting a DC current is not connected), no DC current flows in the head 10.
In the above changeover circuit 40, the transistors Q41 and Q42 are in an ON state during a reproducing process. However, when the ON resistances thereof are large, the transistors are easily affected by hum noise. For this reason, the ON resistance thereof must be decreased.
In order to decrease the ON resistances, transistors each having a large current amplification factor hFE are preferably used as the transistors Q41 and Q42. The transistor having a large current amplification factor hFE is expensive, so that the cost of the tape recorder increases.
Therefore, a current I40 flowing through the transistors Q41 and Q42 may be increased to decrease the ON resistances thereof.
The current I40 flows from the terminal T41 into the transistors Q41 and Q42 through the transistor Q43 and respective resistors R41 and R42. The current I40 further flows from the transistors Q41 and Q42 into the IC 20 through the terminal T23 to be a load on the voltage forming circuit 24. At this time, the voltage forming circuit 24 is originally used to provide the reference voltage VBIAS to the internal circuits of the IC 20, and is constituted without considering a current flowing thereinto from the terminal T23. Therefore, when the current I40 flows into the terminal T23 and its increases, the voltage VBIAS at the terminal T23 rises.
When the voltage VBIAS rises, in the IC 20, the operating points of the circuits 21 and 22 and the like using the voltage VBIAS as a reference voltage or a bias voltage are shifted, and hence malfunction occurs. Therefore, the ON resistances of the transistors Q41 and Q42 cannot be decreased by increasing the current I40.